1. Field of the Invention
The present invention relates to a method for operating a transmitting/receiving circuit as well as to a device for implementing the method. More specifically, the present invention is directed to a method and apparatus for receiving a reflected ultrasonic signal free from interferences.
2. Description of the Prior Art
It is known to modulate a relatively high frequency matched to the inherent frequency of an ultrasonic transducer according to an arbitrary modulation method and to radiate the frequency after amplification by the ultrasonic transducer. The ultrasonic echo reflected from an object to be evaluated may be received by the same transducer and can be evaluated after an amplification. Commonly the elapsed time between the transmitted and received signal is measured, whereat the signal detection may be done on the rising or trailing edge of the modulated signal.
With a frequency selective device, as for instance a narrow band filter within the receiving circuit, freedom of interferences with respect to the received signal may be importantly increased. Air-coupled sonic transducers have a relatively high functional relationship of temperature with respect to their inherent frequency. Because of this functional relationship of temperature of the frequency radiated by the transducer the selective frequency of the filter has to follow the transmitted frequency. This may be done in a manner as it is shown and described in the prior art wherein the frequency of a voltage-controlled oscillator within a phase-locked loop is readjusted, i.e., a larger frequency range is passed through, in which range the received frequency is located anywhere so that at reaching of the pull-in range, the phase-locking loop is locked.